Structural Health Monitoring recent issues

Sensitivity Enhancement of Long-gage FBG Sensors for Macro-strain Measurements

Suzhen Li, , Zhishen Wu, — Tue, 13 Oct 2009 02:29:50 PDT

In our recent study, a structural health monitoring strategy based on distributed fiber optic sensing techniques has been proposed to utilize the strain responses throughout the full or some partial areas of structures to detect the arbitrary and unforeseen damage. However, to perform this strategy more effectively, there is an increasing demand for improving the ability of such sensors to measure small structural responses, especially for the cases such as damage identification based on ambient vibration tests, fatigue crack monitoring of steel structures and crack detection of reinforced concrete structures. This work puts forward a novel packaging design for long-gage fiber Bragg grating (FBG) sensors to enhance the measurement sensitivity of strain responses. The basic idea is to utilize two materials of different stiffness to package the in-tube optical fiber and impose the deformation within the gage length largely on the short-gage sensing part of FBG. Proof-tests indicate that the measurement sensitivity of the innovative FBG sensor can be artificially enhanced by adjusting the two recoating materials and their respective lengths. A series of tests are carried out to verify the ability of the improved sensors to detect small crack and measure slight vibration.

Three-point Frequency Tracking Method

Pai, P. F. — Tue, 13 Oct 2009 02:29:50 PDT

This paper presents and evaluates in detail a harmonics tracking method (HTM) for tracking the instantaneous frequency and amplitude of a vibration signal by processing only three most recent data points. Teager—Kaiser algorithm (TKA) is a popular four-point method for online frequency tracking, but its accuracy is easily destroyed by measurement noise due to the use of finite difference. Moreover, because a signal is assumed to be a pure harmonic in TKA, any moving average in the signal can destroy the accuracy of TKA. On the other hand, HTM uses a constant and a pair of harmonics to fit three recent data points and estimate the instantaneous frequency and amplitude, and it dramatically reduces the influence of any moving average. Moreover, noise filtering is an implicit capability of HTM even if only three points are processed, and this capability increases with the number of processed points. However, HTM depends on TKA to provide the first frequency estimation in order to start online tracking. To compare HTM and TKA and evaluate the accuracy of HTM, Hilbert— Huang transform (HHT) is used to extract accurate time-varying frequency and amplitude by processing the whole data set without assuming the signal to be harmonic. Frequency and amplitude tracking of different amplitude- and/or frequency-modulated signals and nonlinear nonstationary dynamic signals is studied. Results show that HTM is more accurate, robust, and versatile than TKA for online frequency tracking. Moreover, the frequencies and amplitudes tracked by HTM have about the same accuracy as those extracted by HHT, but without the edge effect that HHT suffers from.

Assessment of Vibration-based Damage Identification Methods Using Displacement and Distributed Strain Measurements

Philips Adewuyi, A., Wu, Z., Kammrujaman Serker, N.H.M. — Tue, 13 Oct 2009 02:29:51 PDT

Accurate measurement and monitoring of vibration characteristics is critical for proper detection of the location and severity of damage. However, experimentally measured modal properties are inevitably corrupted by measurement noise and errors, which could render most vibration-based structural damage identification algorithms unreliable for civil structural health monitoring (SHM). This article, through computer simulation and experimental investigation of a simply supported beam, comparatively evaluates the performance of these techniques for practical civil SHM by using displacement modes from accelerometers and long-gage distributed strain measurements. Most of all the techniques proved unreliable for damage identification using noisy measurements from accelerometers, while successful with distributed strain measurements. The findings reveal that long-gage distributed strain measurements are much more efficient choice over the traditional measurement techniques for reliable civil SHM. It may therefore be concluded that the performance of some algorithms might be improved for application to civil infrastructure by using distributed strain fiber optic sensing measurement techniques.

Time Domain Reflectometry Automatic Bridge Scour Measurement System: Principles and Potentials

Yu, X., Yu, X. — Tue, 13 Oct 2009 02:29:51 PDT

Bridge scour is a major factor contributing to the failures of highway bridges constructed over waterways. During flood events, sediments can be washed away and bridge substructures (piers and abutments) are left inadequately supported. Field monitoring of bridge sour process is necessary to study the scour mechanism, to develop scour-resistant design of bridge piers and abutments, to implement effective scour countermeasures, and to deploy safety warning systems. The current instruments are not completely satisfactory in providing real-time monitoring data during critical flood events. This study introduces the development of an automatic scour monitoring system using time domain reflectometry (TDR) principle. It presents the principles and system design of the TDR sediment scour monitoring system. An analyses algorithm for scour signals has been developed. It was found to be robust and can be implemented to automate scour signal interpretation. Simulated experiments were conducted to validate the performance of the scour monitoring system prototype. The results showed that this TDR technology can accurately measure the scour depth. Besides, the properties of the sediments, such as the porosity and density can be estimated with reasonable accuracy. At the end of the paper, a few important issues associated with field deployments of TDR scour monitoring system are discussed.

Onset of Resin Micro-Cracks in Unidirectional Glass Fiber Laminates with Integrated SHM Sensors: Experimental Results

Ghezzo, F., Huang, Y., Nemat-Nasser, S. — Tue, 13 Oct 2009 02:29:51 PDT

This article presents the results of experiments conducted in order to identify and locate the failure initiation in glass fiber/epoxy laminates with integrated structural health monitoring sensors (SHM) and electronics. Recent advances in health monitoring technologies have resulted in the development of micro-dimensional devices that can be embedded into composite laminates. Notwithstanding their small size, such inclusions may affect the response of the composite. Damage induced by the peak values of stress concentration around the embedded inclusion is, in fact, one of the main concerns in smart structures technology. To address this specific issue, unidirectional S2 glass fiber/epoxy laminated composites are fabricated with embedded small implants that mimic potential sensors and microprocessors. Quasi-static tensile tests are then performed on those samples while monitoring them by the acoustic emission (AE) technique. Additionally, the microstructure of the material with and without implants is explored. The AE results show that early low-medium amplitude events are detected at the implant location and the micrographic inspections reveal that micro-cracks initiate at the device-composite matrix interface and grow around the implant causing the debond of the external component from the surrounding resin system.

Onset of Resin Micro-cracks in Unidirectional Glass Fiber Laminates with Integrated SHM Sensors: Numerical Analysis

Huang, Y., Ghezzo, F., Nemat-Nasser, S. — Tue, 13 Oct 2009 02:29:51 PDT

The embedment of micro-sensors and micro-devices into composite laminates for structural health monitoring systems leads to stress/strain concentrations due to geometrical and material discontinuities around such embedded inclusions, with high potential to initiate premature failures. This article presents the efforts to estimate the effects of these stress/strain concentrations induced by the integration of rectangular-shape sensors within unidirectional fiber-glass composites. The micro-crack initiation sites and the failure load are predicted using finite-element simulations. Good agreement has been found between the numerical results and the experimental findings presented in an accompanying paper.

Development of a Real-time Remote Structural Monitoring Scheme for Civil Infrastructural Systems

Tue, 13 Oct 2009 02:29:51 PDT

Long-term structural monitoring has become an important requisite to ensure structural and operational safety for critical civil infrastructures. Long-term structural monitoring by acquiring data continuously or at small intervals may be difficult to achieve by employing conventional on-site monitoring methods. In order to overcome these difficulties remote structural monitoring (RSM), an advanced structural monitoring technique, can be used to acquire continuous data from the remotely instrumented structure. RSM methodology requires an interdisciplinary approach integrating areas such as structural engineering, sensor technology, communication technology, statistical mechanics, information technology for online data transmission (over larger distances) and damage detection/ health assessment of the structure. RSM has many advantages like, continuous monitoring, early alarm of any incipient damage, and data acquisition even in adverse climatic/environmental conditions. This article describes the development of an RSM scheme, developed at SERC, Chennai. The article outlines the laboratory and field investigations carried out to validate the developed scheme. Brief information about the observations and modifications made during these trials are also presented. A part of the work carried out for synthesizing online data using Auto Regressive Moving Average model is also presented.

Correlating Low-energy Impact Damage with Changes in Modal Parameters: A Preliminary Study on Composite Beams

Shahdin, A., Morlier, J., Gourinat, Y. — Tue, 13 Oct 2009 02:29:51 PDT

This article is an experimental study of the effects of multi-site damage on the vibration response of a composite beam damaged by low-energy impact. The variation of the modal parameters with different levels of impact energy and density of impact is studied. Specimens are impacted symmetrically in order to induce a global rate of damage. A damage detection tool Damage Index is introduced in order to verify the estimation of damping ratios. Design of Experiments is used to establish the sensitivity of both energy of impact and density of damage. The DOE analysis results (using natural frequency only) indicate that impact energy for 2nd, 3rd, and 4th bending modes is the most significant factor contributing to the changes in the modal parameters for this kind of symmetrical dynamic test.

Structural Health Monitoring of a Metallic Sandwich Panel by the Method of Virtual Forces

White, J. R., Adams, D. E., Jata, K. — Tue, 13 Oct 2009 02:29:51 PDT

A structural health monitoring system for assessing thermal damage in a metallic sandwich panel was developed in this article. The method of virtual forces was utilized to detect, locate, and quantify damage. The method assumes that a damaged structural response is a summation of the undamaged response and the response from an effective forcing function, which represents the effects of the material damage. The effective forcing function, or virtual force, is an estimate of the forces that the damage mechanism exerts on the undamaged structure. Virtual forces were shown to detect changes in the mass, stiffness, and damping matrices in a lumped parameter dynamic model. A finite element model of a mechanically attached sandwich metallic panel was used to detect a stiffness loss at different locations between two sensor locations. These results were then compared with experimental results involving thermal debond between the face sheet and core of an Al—Al honeycomb panel. Virtual forces indicated there was damage associated with magnitude and frequency shifts in the frequency response function measurements made on the damaged panel. Damage was located relative to the sensor/actuator measurement locations. Damage that was not adjacent to measurement locations was less accurately located and quantified. Variability in producing localized thermal debond using an acetylene torch was also apparent in the virtual force estimates. The virtual force estimates also indicated that damage was spread over several measurement locations because highly localized thermal debond was difficult to produce.

Experimental Investigation on Statistical Moment-based Structural Damage Detection Method

Xu, Y.L., Zhang, J., Li, J.C., Xia, Y. — Tue, 13 Oct 2009 02:29:51 PDT

Although vibration-based structural damage detection methods have demonstrated various degrees of success, the damage detection of civil structures still remains as a challenging task. The main obstacles include the insensitivity to local damage and the high sensitivity to measurement noise. A new structural damage detection method based on the statistical moments of dynamic responses of a structure has been recently proposed by the authors, and the numerical study manifested that the proposed method is sensitive to local structural damage but insensitive to measurement noise. The experimental investigation on this method is presented in this article. Three shear building models with and without damage were built and subjected to ground motions generated by a shaking table. The displacement and acceleration responses of each building model at each floor were recorded. The recorded ground motion and building responses as well as identified structural damping ratios were then used to identify damage locations and severities using the statistical moment-based damage detection method. The identified damage locations and severities were compared with the theoretical values. The comparison is found satisfactory, and the method proposed is effective and feasible.

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Damage Detection of Shear Connectors in Composite Bridges

Liu, K., De Roeck, G. — Thu, 27 Aug 2009 08:10:29 PDT

Headed shear studs are commonly used to resist longitudinal shear forces in composite railway bridges. Due to the growth of traffic and increase in train speed, these studs are subjected to high-cycle fatigue loading which may lead to damage, thus affecting the integrity between the steel girder and the concrete slab. Therefore, it is necessary to find a corresponding nondestructive damage detection method. Within the frame of this paper, the occurrence of damage in shear studs is studied by numerical analysis. In the numerical model of a real composite bridge, headed shear studs are represented by spring elements. A damage indicator based on the local modal curvature and the wavelet transform modulus maxima is proposed for stud damage identification. The efficiency of the damage indicator is investigated by means of numerical simulations where different levels of damage are introduced to the stud by decreasing the spring stiffness. It is verified that the proposed damage index can be used to locate and to quantify the damage.

Passive Impact Location Estimation Using Piezoelectric Sensors

Guyomar, D., Lallart, M., Monnier, T., Xingjun Wang, , Petit, L. — Thu, 27 Aug 2009 08:10:29 PDT

As part of Structural Health Monitoring (SHM), the history of a structure has become a crucial element to take into account. This has been shown, for example, by the spectacular accident of the flight Aloha 243 near Hawaï, when a whole part of the fuselage of a Boeing 737 had been torn off. Thus, monitoring impacts has become particularly interesting to give a comprehensive view of the occurrence of structural damage. Typical impact location estimation techniques use structural frequency drifts of a structure. Thus, such methods need an external excitation of the structure, which is unrealistic in most of the cases. As well, huge and possibly long computations, as genetic algorithms or artificial neural networks, are required for such techniques in order to retrieve the impact location. The scope of the this article was to present a passive impact location estimation using piezoelectric elements for a 1D infinite beam. The principles of the proposed technique was the comparison of the vibrational energy extracted by each sensor. From this comparison, the impact location was cost-efficiently estimated. Theoretical development and experimental results showed that this extraction-based force location estimation method performed well, while being very simple.

Mesh-free Modeling of Ultrasonic Wave Fields in Damaged Layered Half-spaces

Das, S., Kundu, T. — Thu, 27 Aug 2009 08:10:29 PDT

Modeling of an ultrasonic wave field inside a layered half-space is carried out by using the mesh-free semi-analytical Distributed Point Source Method (DPSM). The complete field is computed in a layered half-space in presence and absence of defects. The layered structure is excited by a bounded ultrasonic beam generated by a finite-sized transducer. It is important to have theoretical models to predict the ultrasonic fields in damaged and damage-free structures for its nondestructive evaluation. Numerical exercises can be carried out aided by these theoretical models to determine the area of the most distorted ultrasonic field in presence of an internal anomaly. Several numerical examples are provided for an aluminum half-space attached to layers made of two different materials. The structure is excited by a bounded ultrasonic beam. Influence of the material properties and internal anomaly on the ultrasonic field pattern is demonstrated here.

Potential and Limitations of a Deconvolution Approach for Guided Wave Structural Health Monitoring

Cicero, T., Cawley, P., Simonetti, F., Rokhlin, S.I. — Thu, 27 Aug 2009 08:10:29 PDT

Ultrasonic guided waves offer the possibility of inspecting large areas of structures from a small number of sensor positions. However, inspection of complex structures is difficult as the reflections from different features overlap. Estimating the number and amplitude of the wave packets in ultrasonic time traces is crucial for the development of a guided wave inspection system, in order to detect and locate damage. Deconvolution has been extensively used in geophysical applications to resolve overlapping echoes in the recorded signals. The main objective of this work was to evaluate the applicability of the deconvolution approach for enhancing the resolution of ultrasonic time traces in structural health monitoring (SHM). Numerical simulations on strongly overlapping signals were carried out to evaluate the performance of the two techniques that have been considered: (i) Wiener filter, (ii) single most likely replacement. It was shown that the relatively narrow bandwidth of the input signals and phase shifts between different reflections limit the benefits obtained from deconvolution and it was concluded that deconvolution is unlikely to be useful for guided wave SHM applications.

Numerical and Experimental Studies of a Substructural Identification Strategy

Tee, K.F., Koh, C.G., Quek, S.T. — Thu, 27 Aug 2009 08:10:29 PDT

For structural health monitoring it is impractical to identify a large structure with complete measurement due to limited number of sensors and difficulty in field instrumentation. Furthermore, it is not desirable to identify a large number of unknown parameters in a full system because of numerical difficulty in convergence. A novel substructural strategy was presented for identification of stiffness matrices and damage assessment with incomplete measurement. The substructural approach was employed to identify large systems in a divide-and-conquer manner. In addition, the concept of model condensation was invoked to avoid the need for complete measurement, and the recovery process to obtain the full set of parameters was formulated. The efficiency of the proposed method is demonstrated numerically through multi-storey shear buildings subjected to random force. A fairly large structural system with 50 DOFs was identified with good results, taking into consideration the effects of noisy signals and the limited number of sensors. Two variations of the method were applied, depending on whether the sensor could be repositioned. The proposed strategy was further substantiated experimentally using an eight-storey steel plane frame model subjected to shaker and impulse hammer excitations. Both numerical and experimental results have shown that the proposed substructural strategy gave reasonably accurate identification in terms of locating and quantifying structural damage.

Structural Damage Detection Using Modal Curvature and Fuzzy Logic

Chandrashekhar, M., Ganguli, R. — Fri, 19 Jun 2009 08:01:31 PDT

A fuzzy logic system (FLS) with a new sliding window defuzzifier is proposed for structural damage detection using modal curvatures. Changes in the modal curvatures due to damage are fuzzified using Gaussian fuzzy sets and mapped to damage location and size using the FLS. The first four modal vectors obtained from finite element simulations of a cantilever beam are used for identifying the location and size of damage. Parametric studies show that modal curvatures can be used to accurately locate the damage; however, quantifying the size of damage is difficult. Tests with noisy simulated data show that the method detects damage very accurately at different noise levels and when some modal data are missing.

Lead-free Piezoelectric KNN-based Pin Transducers for Structural Monitoring Applications

Lam, K.H., Lin, D.M., Ni, Y.Q., Chan, H.L.W. — Fri, 19 Jun 2009 08:01:31 PDT

A 2.4 mm diameter piezoelectric ceramic disc was used to construct a spring-loaded pin-type transducer for intelligent monitoring of systems. Two types of lead-free KNN-based ceramics (0.95K_0.5Na_0.5 (Nb_0.94Sb_0.06)O₃ — 0.05LiTaO₃ + 1.25 mol% BaO and K_0.5Na_0.5NbO₃ -0.75 mol% K_5.4Cu_1.3Ta₁₀O₂₉) were selected as a sensing element of the pin. The piezoelectric spring-loaded pins were mounted on the corners of the aluminum plate for the performance evaluation using a laser ultrasonic technique. The signals detected by transducers with different sensing elements including lead-based ceramics have been compared. The capability of impact detection of the lead-free transducers was shown to be comparable to that of the lead-based one. It is envisaged that lead-free piezoelectrics will become the important transducer materials in the next generation nondestructive evaluation technology.

An Investigation on Vibration-based Damage Detection in Circular Plates

Trendafoilova, I., Gorman, D.G., Manoach, E. — Fri, 19 Jun 2009 08:01:31 PDT

This study aimed at the development of vibration-based health monitoring methodology for thin circular plates. The possibility of using the first several natural frequencies of a circular plate for damage detection purposes was investigated first. The study then suggested a damage detection method, which considered a vibrating plate as a dynamic system and used its time-domain response represented in a new phase (state) space to extract damage sensitive characteristics. The paper introduced the idea of using large amplitude vibrations and nonlinear time series analysis for damage detection purposes. The suggested damage detection approach explored the possibility to use certain characteristics of the distribution of phase space points on the attractor of the system. It studied the histograms of this distribution and attempts to extract damage sensitive features. Three damage features were suggested and they are shown to detect damage at a rather low level using a finite element model of the plate. The method suggested was rather generic and permits development and application to more complex structures and real data.

Robust Damage Metric in Terms of Magnitude and Phase for Impedance-based Structural Health Monitoring

Xianhua Liu, , Paurobally, R. — Fri, 19 Jun 2009 08:01:31 PDT

Structural health monitoring has been a research interest for its great potential for life safety and economic benefits for decades. Structural vibration impedance by way of piezoceramic patch excitation and sensoring offers a local damage detection technique and has caused a wide research interest. The commonly used damage index in this method is the rate of change of the real part of the measured electromechanical impedance. This paper studied several damage indices constructed by the real and imaginary parts or magnitude and phase. It theoretically deducted and concluded that the damage index in terms of changes in real part is in fact not a properly defined index and is physically obscure; on the other hand, indices in terms of the change in magnitude and phase were shown to be physically clear and properly defined. It further verified that a new damage index in terms of the whole complex-valued impedance turned out to be the elegant combination of the magnitude and phase damage indices in the form of their Euclidean norm. Experimental result further demonstrated that the damage index in terms of real part was largely influenced by the phase, whereas the whole complex impedance offered an optimized and robust damage metric.

Near-field Sub-band Beamforming for Damage Detection in Bridges

Medda, A., DeBrunner, V. — Fri, 19 Jun 2009 08:01:31 PDT

Traditional structural health monitoring techniques based on the vibration response of bridge structures are limited because of several factors — including a poorly formed aggregate system model, very low SNR, and unrealistic boundary conditions. Many times, these methods rely on global parameters to describe the dynamic behavior of local structural elements. In this paper, we proposed a novel efficient SHM technique that employs the use of compactly supported sub-band space/ frequency and time/frequency analysis using local vibration characteristics. To overcome the problem of the low-error sensitivity of features extracted from vibration signals, a near-field adaptive beamforming approach was used. This technique allows the sensor array to `scan' local portions of the structure, resulting in accurate spatial selectivity on the array and high signal-to-noise ratio for any given scan direction. Moreover, the sensor array was in direct contact with the vibrating structure, and thus the measured source is in the near-field of the array. Therefore, compensation of the sensor output was needed. Sub-band analysis and adaptive beamforming were integrated in a wavelet packet sub-band framework. We utilized the 3D energy map of the optimized sub-band signals for any given scan direction and for each sub-band center frequency as the damage detection feature. We validated our method using classical finite elements approximation to the dynamic behavior of the system. The comparison of a simulated undamaged simply supported beam with a damaged equivalent revealed that damage is localized in frequency and aligned with the direction of the simulated damage. The energy signature comparison after the beamforming stage validates these results, localizing the damage in areas of high probability around the direction of the damage. Automatic analysis of the energy comparison map was possible using a pseudo density estimation of the damage location, allowing for an automatic damage detection procedure. The focus of our research was aimed toward the adaptation of this damage detection method to real highway bridges.

An Oil Condition Monitoring Technique to Determine the Optimal Oil Type and Maintenance Schedule

Ahmadi, H., Mollazade, K. — Fri, 19 Jun 2009 08:01:31 PDT

Oil analysis is the main condition monitoring technique for reciprocating machinery maintenance and fault diagnosis. The object of this research was to choose and to investigate the best oil for Dump Truck HD325-5, used for transportation of minerals, by oil condition monitoring. This was achieved by analysing different oil samples after use in Dump Truck HD325-5. The oil analyses were initially carried out at regular intervals during life. A series of tests were then conducted during the operating hours of the machine. Oil samples were regularly collected. Numerical data produced by oil analyses were compared with those from another sample in order to quantify the effectiveness of the results of oil condition monitoring technique. The results from this article have enhanced understanding on the dependent and independent roles of oil analyses in predicting which oil is more suitable for a machine in working condition.

Wireless Sensor Networks for Strain Monitoring during Steel Bridges Launching

Chacon, R., Guzman, F., Mirambell, E., Real, E., Onate, E. — Tue, 14 Apr 2009 08:25:20 PDT

In this paper, an experimental test performed on a hybrid steel plate girder was subjected to concentrated loads at the end of an unstiffened panel is presented. This load was intended to produce a typical reaction of the piers while launching steel girders in bridges. In this test, strain measurements were taken with conventional pre-wired gauges as well as with newly developed wirelessly connected strain-measuring system. Both measurements were carefully compared and the accuracy of the developed system was demonstrated. Results were also compared with numerical simulations deployed with FE models and on such a basis; the reliability of the developed wireless system was proven. Finally, suggestions of potential research trends in this area are provided.

Localized Damage Detection of Structures Subject to Multiple Ambient Excitations Using Two Distance Measures for Autoregressive Models

Haitao Zheng, , Mita, A. — Tue, 14 Apr 2009 08:25:21 PDT

In this paper, the distance measures of autoregressive (AR) models are used as damage indicators. Two distance measures are discussed: one is the Itakura distance, and the other is the cepstral distance. The distance measures of AR model have been successfully applied in image, speech, and neurological signal processing applications. This research explores new applications of two distance measures for damage detection in civil engineering. A five-storey building model is used for performance verification. Verification simulations show efficiencies of both distance-based damage indicators when the excitations are mutually uncorrelated. However, the ability of damage indicators for damage localization is deteriorated when the multiple excitations are mutually correlated as there are strong correlations among them. In practice, the excitations acting on civil engineering structures are mutually dependent and correlated, such as wind and traffic loading. To overcome this difficulty, a pre-whitening filter is applied for removal of correlations in excitations before calculating the damage indicators. To examine the proposed methodology, simulation and experiment data have been tested. It can be concluded from the results that, by using the pre-whitening filter, the damage identification ability of the proposed damage indicators improves significantly, especially for damage localization. The damage indicators increase monotonically with damage severity, which provides the potential to damage quantification.

On Selection of Data Fusion Schemes for Structural Damage Evaluation

Zhongqing Su, , Xiaoming Wang, , Li Cheng, , Long Yu, , Zhiping Chen, — Tue, 14 Apr 2009 08:25:21 PDT

Data fusion plays a pivotal role to achieve reasonable accuracy and precision in identifying structural damage. An appropriate fusion process can reduce imprecision, uncertainties and incompleteness, therefore increasing the robustness and reliability of identification. The present work compared three major fusion schemes, i.e., disjunctive, conjunctive, and compromise fusion, in terms of their effectiveness to estimate mono- and multi-delamination in carbon fiber-epoxy composite structures. (1) Time-of-flight was extracted from Lamb wave signals rendered by an active sensor network, to attain the loci of locations of all possible damage instance(s) in the structure under inspection, which served as the prior perceptions of sensors as to the areas with possibility of damage occurrence; and (2) the entire structure was virtually meshed and the prior perceptions of individual sensors were further quantified at each spatial mesh node using the distance between nodes and all loci established from (1), to form prior probabilities of damage occurrence at nodes. Then, three fusion schemes were employed to fuse the prior probabilities at all spatial nodes to shape a posterior consensus concerning the overall health status of the structure. Hybrid fusion by combining three basic schemes was also explored. Conclusions drawn from this study have given an indication on how to select a better, if not the most optimal, data fusion scheme for structural damage evaluation.

Vertical Displacement Measurements for Bridges Using Optical Fiber Sensors and CCD Cameras -- A Preliminary Study

Tue, 14 Apr 2009 08:25:21 PDT

Bridge managers all over the world are always looking for simple ways to measure bridge vertical displacements for structural health monitoring. However, traditional methods to obtain such data are either tedious or expensive. There is a need to develop a simple, inexpensive, and yet practical method to measure bridge vertical displacements. This paper proposes two methods using either optical fiber (FBG) sensors or a charge-coupled-device (CCD) camera, respectively, for vertical displacement measurements of bridges. The FBG sensor method is based on the measured horizontal strains together with the identified curvature functions obtained by a self-developed FBG Tilt sensor. CCD cameras use a large number of pixels to form an image. The CCD camera method utilizes image processing techniques for pixel identification and subsequent edge detection. A preliminary study to validate the proposed methods in laboratory was presented. The tests include applying the methods to determine the vertical displacements separately for a concrete beam and a steel beam under various loadings. The comparisons include their installations, costs, degrees of accuracy, external factors affecting the measurement, etc. It was concluded that both methods could be used for vertical displacement measurement, and they could be complementary with one another. It was suggested to further improve the two methods developed and a successful outcome will not only help to solve an important problem for bridge management, but also prepare the way for better structural health monitoring techniques.

A Nonlinear Acoustic Technique for Crack Detection in Metallic Structures

Dutta, D., Sohn, H., Harries, K. A., Rizzo, P. — Tue, 14 Apr 2009 08:25:21 PDT

A crack detection technique based on nonlinear acoustics is investigated in this study. Acoustic waves at a chosen frequency are generated using an actuating lead zirconate titanate (PZT) transducer, and they travel through the target structure before being received by a sensing PZT wafer. Unlike an undamaged medium, a cracked medium exhibits high acoustic nonlinearity which is manifested as harmonics in the power spectrum of the received signal. Experimental results also indicate that the harmonic components increase nonlinearly in magnitude with increasing amplitude of the input signal. The proposed technique identifies the presence of cracks by looking at the two aforementioned features: harmonics and their nonlinear relationship to the input amplitude. The effectiveness of the technique has been tested on aluminum and steel specimens. The behavior of these nonlinear features as crack propagates in the steel beam has also been studied.